Health care reform legislation has significant implications for access to services and expansion of benefits for those with substance use disorders (SUD), particularly those with HIV. These conditions are associated with multiple medical and psychiatric comorbidities as well as high health care costs. Responding to RFA-DA-13-001, the proposed study focuses on key elements of the Affordable Care Act (ACA) that will be implemented in 2014 and will likely increase demand for SUD and HIV treatment services. The study evaluates the impact of the ACA on individuals with SUDs in a care setting well-suited to ACA implementation research. Kaiser Permanente Northern California (KPNC) is a large health care system providing integrated SUD and medical treatment and has been designated as the statewide benchmark for small group plans in the California health insurance exchange. Our innovative approach is informed by an integrated conceptual model of organizational theory and health service utilization. In this phased study, the Aims of the R21 phase lay the groundwork for analyzing changes in membership demographics and clinical complexity, services use and costs, using a cohort (n=3641) of newly enrolled pre-ACA health plan members with SUDs, including members with both SUDs and HIV. In the R33 phase, we use a pre-post design to compare the R21 cohort to a second, post-ACA cohort of newly enrolled KPNC members with SUDs, using advanced difference-in-difference methods to analyze ACA-related changes that begin in 2014. Key outcomes include membership changes, health services utilization and costs, SUD treatment initiation and retention, and HIV management. The post- ACA cohort will also be followed longitudinally over two years. With an innovative mixed-methods approach, we use data from the health plan's extensive electronic medical record, supplemented by qualitative interviews with organizational and clinical leaders regarding ACA-related changes to benefit plans, service delivery, workforce, and barriers or facilitators that impact ACA implementation. Findings will be highly informative in understanding how health plans implement the ACA and serve newly insured SUD and HIV patient populations, and will contribute to future health policy.